I'm curious too and after a lot of research I believe it is a little of both.
I raise Black Penedesencas and are equally famous as Poulet de Bresse for meat flavor in its respective region of Europe.. It's claimed their meat is organoleptically unique. That said, I suspect it also has to do with the soil in the region they're grown. Otherwise why would people come from across Europe to buy the birds in Catalonia in the case of the Pene or to the Bresse region to get the Poulets when they could just raise the birds themselves.
I haven't raised enough yet to be able to do a side by side blind taste test. I have a roo I'm butchering in the next week as well as a bunch of Freedom Rangers I'll be butchering in about 6 or 7 weeks so the pene roo will have to go into the freezer first but that may be my first chance to do a comparison.
I just finished an incubator so by February I hope to start hatching lots of eggs and doing some more taste comparisons.